Killer App tells the story of two families whose daughters got involved in online relationships that became a matter of life or death. It's a story that will resonate with millennials, teenagers and parents, who, thanks to so-called "parent proof" platforms, often don't know what their kids are doing on their phones, laptops, tablets or computers until something goes terribly wrong.

A young woman is murdered, police thought her killer was on the run ... or was he? Correspondent Tracy Smith investigates.